This invention relates in general to monitoring and evaluating brain electrical activity, and in particular includes methods and systems for methods for monitoring and evaluating local synchrony between adjacent or neighboring brain regions or neural sites.
EEG, or electroencephalogram, is used to provide a measure of brain electrical activity by monitoring electric potential at multiple locations on the scalp. Synchronicity of EEG waveforms at separate scalp locations can be quantified to provide a measure of synchronicity of two neural groups or regions, which can provide an indication of similarity of behavior of the neural groups or regions. There are several known methods for measuring synchronicity between waveforms, including “coherence,” which is described further below, as well as the article, “Spatial Correlation of the Infant and Adult Electroencephalogram,” by Philip G. Grieve, Ronald Emerson, William P. Fifer, Joseph R. Isler, and Raysmond I. Stark, Clinical Neurophysiology 114 (2003) 1594-1608, which is hereby incorporated herein by reference in its entirety. Synchronicity and coherence measurement can provide an indication of a degree of coordination of neuron activity and function in different brain areas.
Using EEG performed with a large number of scalp locations for electrode placement, it is possible, via coherence measurement, to provide a global quantitative evaluation of apparent relatedness of neural function in disjoint brain regions, such as frontal to occipital brain regions or between hemispheres of the brain. Determining local coherence between brain regions or spatially spaced neuron sites, however, has been impractical.
In theory, EEG measurements could provide a measure of EEG local coherence, a specific synchrony measure. Using EEG to determine local synchrony, however, is fraught with difficulty. Generally, EEG measurements are taken by placing a number of electrodes spaced on an adult subject's scalp. However, if the electrodes are closely spaced, as would be necessary to attempt to obtain EEG information from which local synchrony information might be obtained, volume conduction artifact due to the presence of the skull produces a spatial blurring of electrical variation. Reasons for this include the thickness of the adult skull and its poor electrical conductivity. A result of such artifact and blurring is that closely spaced electrodes on the adult scalp yield data which suggests a high degree of local synchrony, whether or not such local synchrony is present. As such, the measured adult EEG information is itself insufficient to allow determination of local synchrony.
The remaining subsections of this section include detailed discussion of aspects of the above. Subsection (a) includes a discussion of global cortical function and theoretical determination of local synchrony using EEG information. Subsection (b) includes a discussion of EEG in the context of local synchrony between spatially distributed brain neuron sites. Finally, Subsection (c) discusses includes a discussion of how volume conduction obscures local synchrony measurements attempted using prior art EEG techniques.
a. Global Cortical Function and EEG Synchrony
The cerebral cortex is the anatomical structure where the highest level of information processing occurs. It is also the source of the electrical activity of the brain that is manifested as a spatial distribution of electric potential on the scalp known as the electroencephalogram (EEG). The cortex is known to provide distributed processing of neural information in groups of neurons that are connected to other groups that are both neighboring and distant. The electrical activity of a coordinated neural group is conducted to the scalp where it produces the EEG. The EEG is measured via scalp electrodes which are connected to amplifiers. The variation of the EEG voltage over time at a measurement site is characteristic of the neural activity in close proximity to that site. Further, the similarity of behavior (i.e., synchronicity) of EEG voltage versus time waveforms at two separate scalp locations can be quantified and is indicative of the degree of similarity of behavior of the two neural groups that produced the EEG waveforms.
There are well known methods for measuring the similarity of behavior in two waveforms such as the correlation function and the correlation coefficient. Another technique is “coherence” which is a well known signal processing technique that provides a quantitative measure of the frequency dependent correlation of two waveforms, as discussed in “Spatial Correlation of the Infant and Adult Electroencephalogram,” by Philip G. Grieve, Ronald Emerson, William P. Fifer, Joseph R. Isler, and Raysmond I. Stark, Clinical Neurophysiology 114 (2003) 1594-1608 (hereinafter, “Grieve, et al. (2003)”), which is hereby incorporated herein by reference in its entirety. Thus, for a given mental task, if the EEG is measured at a large number of spatial locations and the similarity of behavior over time is calculated for the EEG at one location with EEG waveforms from other locations, it is possible to provide a global (e.g., frontal to occipital or between hemispheres) quantitative description of the apparent relatedness of neural function in disjoint brain regions. This is the essence of a large field of research over the last several decades which has largely been supplanted by fMRI studies.
b. Theoretical EEG Local Synchrony Quantifies Coordinated Local Neural Activity
However, these global neural synchrony measurements do not provide insight into the functional behavior of localized groups of activated neurons. For example, this behavior is of particular interest because the cortex is organized into anatomical columns of neurons that cooperate to perform various functions. The columns are also interconnected laterally with neighboring columns to perform coordinated localized neural processing. Although the anatomical columns are on the order of 1 mm or less in diameter, larger groups of columns performing similar functions can be many cm in dimension. Quantification of local synchrony requires the measurement of the synchronized activity of neurons at a particular spatial site with nearby (e.g., mm to cm) neighboring neural groups.
The EEG theoretically provides a means of making this measurement if we can measure the average degree of synchronicity of the EEG from a particular site with the EEG collected at the immediate neighboring sites surrounding the site of interest. A high average value for local synchrony defined in this manner indicates coordinated neural activity in the local region which may be indicative of functional activation of the neurons. This is comparable to the fMRI BOLD signal which quantifies the local decrease in deoxyhemoglobin which is indicative of neural activation in a region.
c. Volume Conduction Artifact Obscures EEG Local Synchrony
Although it would appear straightforward to calculate local synchrony from EEG data, the volume conduction of current from cortical neurons to the scalp makes the accurate measurement of synchrony difficult when the EEG is measured from closely spaced leads as is required for local synchrony. The major cause of volume conduction is the poorly conducting skull which produces a spatial blurring of cortical electrical potential variation when measured with the scalp EEG. Volume conduction can cause the EEG from two closely spaced electrodes to have a high degree of synchrony even when the neural activities at the two sites are unrelated because each electrode captures electrical activity from neurons located near the other electrode. This “crosstalk” artifact causes an erroneous value for local synchrony to be calculated as it combines true synchrony with a constant component that is a function of the degree of spatial spreading of the EEG rather than being related to neural activity. This artifact component is very large for the thick-skulled adult making local synchrony measured for the adult a large constant value that is insensitive to neural activity. This is not true for the infant with a skull 5-10 times thinner than the adult. For example, at an angular electrode spacing of 0.3 radians, the mean value for the lower 10 percentile of adult coherence is 0.9 while that of the infant is 0.3 (see previously incorporated article. This means that for the adult, measured coherence values are essentially always greater than 0.9 no matter whether the neural activities are related or not. However for the infant this value is 3 times lower so that measured infant coherence values can range from 0.3 to 1.0 and are a more sensitive measure of true cortical coherence. That the adult value is 3 times that of the infant is indicative of the large volume conduction artifact present in the adult local coherence measurement.
In contrast, there is essentially no artifact from volume conduction for leads placed directly on the cortical surface. As most EEG research has been with adult subjects, the large amount of volume conduction artifact present in the adult EEG has lead researchers to believe that synchrony measurements made from closely spaced electrodes are of little value, the major finding being that the level of synchrony decreased as the lead spacing increased. This result is largely caused by volume conduction contamination of the synchrony measurements. However there have been a few investigations of coherence from closely spaced electrodes placed directly on the cortical surface of humans and animals.
For the reasons, there is a need for methods and systems for monitoring and evaluating local synchrony and local coherence between neighboring brain regions or neural sites.